Structure determination and time-resolved Raman spectroscopy of yttrium ion exchange into microporous titanosilicate ETS-4

Abstract

The ion exchange of yttrium, one of the five most critical rare earth elements as outlined by the U.S. Department of Energy, into ETS-4 is a dynamic, multi-step ion exchange process. The ion exchange process was followed using in situ time-resolved Raman spectroscopy, and the crystal structure of the pre-exchange and post-exchange forms were determined by single crystal X-ray diffraction. In situ Raman spectroscopy is an ideal tool for this type of study as it measures the spectral changes that are a result of molecular geometry changes at fast time intervals, even where symmetry and unit volume changes are minimally detected by X-ray diffraction. By tracking the step-wise changes in the peak positions and intensities in the spectra, where we focused primarily on the strong spectral features corresponding to titania quantum wires and three membered-ring bending and breathing modes, molecular models were constructed to explain the changes in the Raman spectrum during ion exchange. The multi-step ion exchange process started with rapid absorption of Y into the Na2 site causing titania quantum wires to kink. After this initial uptake, the exchange process slowed, likely caused by hydration coordination changes within the channels. Next, Y exchange accelerated again during which time the Y site moved closer to the framework $$\mathrm{O^{2-}}$$. Crystal structure of the maximal Y exchanged ETS-4 material were determined, and confirmed the splitting of the Y site. Inductively coupled plasma optical emission spectroscopy was also used to quantify the extent of Y exchange, and to measure if there were indications of titania leaching from the framework.

Introduction

Ion exchange in zeolites and related microporous materials have broad-ranging applications, including water purification (Wang 2010), gas separation (Kuznicki 2001), catalysis (Corma 1997), and heavy cation sequestration (Fu 2011) from aqueous solutions. Some of the most important outstanding areas of research in cation exchange in nanoporous materials are to determine the molecular driving forces and finding ways to model the chemical dynamics of this spontaneous processes. Another area that needs to be addressed is determining the crystal structures of intermediate structural states where ingoing cations change, stress, or deform the crystal structure. These areas of research are now being addressed with new in situ experimental techniques, and our study contributes to that effort. In addition, we are investigating the process of how rare earth elements (specifically yttrium) are exchanged into the nanoporous structure. A better understanding of REE ion exchange would have potential applications in the design of future REE catalysts, materials for novel gas separation technology, and pharmaceuticals. One particularly beneficial application is the possibility use of microporous minerals and materials in selective sequestration of rare earth elements from aqueous solutions (Trigueiro 2002).

The rare earth elements (Sc, Y, and the lanthanides) (Damhus 2005) are used in a large number of technological applications (Alonso 2012, Long 2012, Eliseeva 2011), where Y is among the five most critical REE (others are Nd, Eu, Tb, Dy) according to the U.S. Department of Energy (Bauer 2010). The complex nature of REE separation starts at mineral ore separation (Jordens 2013) followed by element separation. Current methods of REE separation are summarized elsewhere (Binnemans 2013, Krishnamurthy 2004, Zhang 2016, Liao 2005, Thomas 1970, Xie 2014).

The use of REE in technology and chemical processing is increasing (Alonso et al., 2012), and there is a relative scarcity of economic ore deposits in the United States. The Molycorp Mountain Pass Mine in California was the only commercial REE mine in the United States where the mineral bastnäsite was the primary REE ore mineral (current closed to mining operations as of November 2015). These factors drive interest to further study the crystal chemistry and ion exchange mechanisms of REE sequestration as a possible way of developing ion selective absorbents, or enhanced catalytic materials, for the mining, recycling, or environmental industries.

The similar physiochemical properties of the REE (e.g. electronic configuration, similar ionic radii, coordination geometry in crystals, hydration spheres in aqueous solutions, and valence states (Gschneidner et al., 2002)) leads to difficulty in their separation. The focus of this study is to investigate ion exchange mechanisms of REE, specifically Y, into the ETS-4 structure. The goal is to better understand the ion exchange mechanisms of high charge density cations into a known functional microporous materials for future material science applications.

Zorite \citep[][]{chukanov2005heterosilicates} ($$\mathrm{Na_{6}Ti_{5}[Si_{12}O_{34}](O,OH)_{5}\cdot 11H_{2}O}$$) is a sodium titanium silicate mineral found at the Lovozero Massif, Kola Peninsula, Russia. The zorite microporous framework is topologically identical to the Ca-rich haineaultite (the Ca-analogue form), chivruaiite (Na and Ca member), and the synthetic zorite analogue Na-ETS-4 (Na member, Englehard Titaium Silicate). The ion selective separation properties of the Na-ETS-4 member have been well studied since its initial development at the Englehard Coorporation \citep{tomita2004gas,kuznicki2001titanosilicate,marathe2004effect,tsapatsis2002molecular}. The efficiency and affinity of Na-ETS-4 to selectively absorb Sr$${}^{2+}$$, which promotes selective gas separation and catalysis, has led to many experimental investigations into its ion exchange properties \cite{braunbarth2000structure,guan2001synthesis,guan2005photocatalytic,pavel2002synthesis,ferreira2009cadmium,popa2006purification}. However, there are very few studies of the exchange of high valent cations into this material, nor have there been detailed studies of the time-dependent changes during ion exchange in ETS-4. The work presented in this study helps to explain how high valent cations diffuse into porous titanium silicates, and has broader application to material design research.

\label{section:experimental}

Synthesis of Na-ETS-4

The sodium-form of ETS-4 was synthesized as follows. In 250 mL polypropylene graduated bottle and stirring with magnetic stir plate, 4.7 mL deionized $$\mathrm{H_{2}O}$$, 1.35 mL $$\mathrm{C_{12}H_{28}O_{4}Ti}$$ were added. Next, 10 mL hydrogen peroxide was added to the stirring mixture, and then 30 mL of deionized $$\mathrm{H_{2}O}$$ was quickly added. After adding the $$\mathrm{H_{2}O}$$  2.75 mL of 10 M $$\mathrm{NaOH}$$ were added. This mixture was allowed to age for 1hour prior to loading in autoclaves. 12 mL of this gel was added to each 23 mL Teflon-lined Parr autoclaves. To each autoclave, 1 mL of AS-40 (40 wt. % amorphous silica) was added. Finally, 1 mL of $$\mathrm{H_{2}O}$$ was added to each autoclave, which were then sealed and placed in oven at 210 $$\mathrm{{}^{\circ}}$$C for two days (48 hours). X-ray powder diffraction confirmed a pure product of Na-ETS-4. Crystal sizes averaged 5 $$\mu$$m $$\times$$ 10 $$\mu$$m $$\times$$ 200 $$\mu$$m.

Inductively Coupled Plasma Optical Emission Spectroscopy

As synthesized and ion exchanged samples of Na-ETS-4 were weighed ($$\approx$$30 mg) and each added to their own platinum dish. Fluxing and wetting agents, approximately 1 g of a lithium tetraborate-lithium carbonate mixture purchased from Claisse and 0.05 g of ammonium iodide purchased from MP Biomedicals, were added to each dish. Samples were heated in a furnace at 1050 $${}^{\circ}$$C for 5 minutes. The mixtures was allowed to cool and then dissolved in 10% trace metal grade hydrochloric acid purchased from Fisher Scientific. Samples were subsequently analyzed using an ICAP 6500 ICP-OES system from ThermoScientific. The extra-framework cations in the Y exchanged Na-ETS-4 material (Y-ETS-4) were measured from three separate analyses to be 7.40 ppm for Na and 13.51 ppm for Y, resulting in a near 50% Y exchange. No trace metals were detected from the Na-ETS-4 analysis. From the ICP-OES data, an approximate chemical formula for Na-ETS-4 and Y-ETS-4 were determined to be $$\mathrm{Na_{6}Ti_{5}[Si_{12}O_{34}](O,OH)_{5}\cdot xH_{2}O}$$ and $$\mathrm{YNa_{3}Ti_{5}[Si_{12}O_{34}](O,OH)_{5}\cdot xH_{2}O}$$, respectively. The $$\mathrm{H_{2}O}$$ and OH content was not determined by ICP.

Ion Exchange

Ion exchange solutions were prepared at room temperature and ambient pressure. The 0.001 M $$\mathrm{YNO{{}_{3}}}$$ solution was made in deionized $$\mathrm{H_{2}O}$$. The low REE concentrations were used to slow the ion exchange process. A vacuum environmental cell (Celestian 2013) was used for the in situ Raman spectroscopy experiments. Suitable single crystals for single crystal X-ray diffraction experiments (approximately 30 to 50 crystals), of Na-ETS-4 were placed inside the cell. The flow rate of the exchanging solution was held constant for the duration of the experiment using a Masterflex peristaltic pump set to 0.05 mL/min. After ion exchange, a suitable single crystal was selected for single crystal XRD.

Raman Spectroscopy

A Thermo DXR dispersive Raman microscope equipped with a 780 nm diode laser (14 mW at the sample), a Peltier-cooled CCD detector, and a high-resolution diffraction grating ($$\simeq$$1.2 cm$${}^{-1}$$ spectral resolution between 50 cm$${}^{-1}$$ and 1800 cm$${}^{-1}$$) was used for all experiments in this study. Calibration of laser frequency and spot position were performed using a polystyrene standard, and the spectrometer was calibrated using the excitation lines from a neon lamp. Exposures for the time-resolved experiment were a total of 30 sec. using the average of three sequential 10 sec. exposures. The microscope and laser settings were held constant for data acquisitions using a 50x (0.5 N.A.) long-working distance objective in orthoscopic geometry (50 $$\mu$$m laser slit), and the spot size of the laser at the sample is estimated to be 2.1 $$\mu$$m. A total of 400 spectra were collected for each experiment, each spectra were smoothed using a Savitzky-Golay algorithm (5 points, 2.411 cm$${}^{-1}$$), and both the original data and the processed data were saved as separate ascii files.

Peak fitting was performed in the Magic Plot software. The advantage of using this software is the flexibility of peak shape profile usage and the ability to batch process the fitting procedure for all spectra. To maintain software stability during the peak fitting process (using a Gaussian profile), the fitting was performed in stages where the spectra were fit in three discrete regions (Figure \ref{fig:figure_fitted-raman-spectrum}). The regions are: Region A is dominated by Si-O-Ti bends, Region B is dominated by pyramidal O-Ti-O and Ti-O-Ti bending, and region C is dominated by octahedral and pyramidal Ti-O stretching modes. Beyond region C is dominated by Si-O stretching modes (Caracas 2011, Mihailova 1996, Nair 2001, Armaroli 2000).

Single Crystal X-ray Diffraction

Single Crystal X-ray Diffraction (SCXRD) data collection were performed on a Bruker AXS Quasar diffractometer using an APEX-II CCD, and structure solution and refinement details for Y-ETS-4 are summarized in Table \ref{table:tab-scxrd} and unit-cells of other ETS-4 materials are compared in Table \ref{table:unit-cells}. For these diffraction experiments, a suitable single crystal was found using a polarized light microscope, where the crystal had good (not undulatory or diffuse) extinction, and was transparent in plane polarized light. The poor scattering statistics of this crystal are due in part to the small size of the crystal, and the amount of disorder in the framework and extra-framework atomic sites. As a result, we were not able to resolve many of the expected $$\mathrm{H_{2}O}$$ positions, and we acknowledge that our estimates of interstitial $$\mathrm{H_{2}O}$$ content and bond valence sums of extra-framework cations are likely underestimated. Crystals structures for the as-synthesized Na-ETS-4 were also determined and were in good agreement with published work (Sandomirskii 1979, Merkov 1973).

Single Crystal X-ray Diffraction

The structure of Y-ETS-4 (Figures \ref{fig:figure_zorite_100}-\ref{figure:local_Y2_Na1}) consists of chains of $$\mathrm{TiO_{6}}$$ octahedra that link 8MR (membered rings) of $$\mathrm{SiO_{4}}$$ tetrahedra. These rings are further linked by disordered five-coordinated $$\mathrm{TiO_{5}}$$ square pyramids. The secondary building units (SBUs) in Y-ETS-4 are 8MR, 7MR, 6MR, and 3MR. Each of these structural aspects of Y-ETS-4 are described in the following sections.

The $$\mathrm{TiO_{6}}$$ chains (Figure \ref{figure:ti-nano-wires}) are oriented parallel to the [100] direction. The kink-angle of the chains is 138.8$${}^{\circ}$$ before ion exchange, which forces the $$\mathrm{O^{2-}}$$ in the $$\mathrm{TiO_{6}}$$ square plane to be adjacent to another $$\mathrm{TiO_{6}}$$ square plane $$\mathrm{O^{2-}}$$ (2.68 Å). After Y ion exchange, the kink-angle of the $$\mathrm{TiO_{6}}$$ chains is slightly smaller, and 137.6$${}^{\circ}$$. These titania chains have been previously referred to as quantum wires, and these wires contain natural defects caused by localized charge-transfer transitions (Yilmaz 2006). The exchange of a high valence $$\mathrm{Y^{3+}}$$ into the crystal structure that is bonded to the $$\mathrm{O^{2-}}$$ in the quantum wires could cause greater defects (with an increase in charge transfer from the REE) in the wire structure as the kink angle in the chains decrease.

Four of the $$\mathrm{SiO_{4}}$$ tetrahedra in the 8MR are disordered with ring dimensions of 7.12 Å, 6.92 Å, and 6.12 Å (Figure \ref{figure:8MR_6MR}). The disordering of the $$\mathrm{SiO_{4}}$$ groups accommodate the disorder of the $$\mathrm{TiO_{5}}$$ linking groups, where $$\mathrm{SiO_{4}}$$ arrangement is either up/up (UU) or down/down (DD). These $$\mathrm{SiO_{4}}$$ 8MR are joined by $$\mathrm{TiO_{6}}$$ chains to form a 3MR SBU containing two $$\mathrm{TiO_{6}}$$ octahedra and one $$\mathrm{SiO_{4}}$$ tetrahedron. Before ion exchange, the bond angle in the 3MR of Ti—O—Si is 135$${}^{\circ}$$, and after Y ion exchange the angle is 136.6$${}^{\circ}$$, accounting for the shorter Y2—O distance of 2.4 Å and 2.6 Å to the $$\mathrm{O^{2-}}$$ of the $$\mathrm{TiO_{6}}$$ group.

A third structural feature are the elliptical 7MR (Figure \ref{figure:ti-nano-wires}) consisting of two $$\mathrm{TiO_{6}}$$ groups, two ordered $$\mathrm{SiO_{4}}$$ groups (parts of the 3MR described above), one $$\mathrm{TiO_{5}}$$, and two disordered $$\mathrm{SiO_{4}}$$ groups. The ordered sites of the $$\mathrm{SiO_{4}}$$ 8MR form a the building units for the 6MR consisting of two $$\mathrm{TiO_{6}}$$ groups and four $$\mathrm{SiO_{4}}$$ groups (Figure \ref{figure:8MR_6MR}). The disordered Na1 site is located in this 6MR ring window, and is bound to six framework $$\mathrm{O^{2-}}$$. The Y2 sites are located in the window of the 7MR and is primarily bound to five framework $$\mathrm{O^{2-}}$$  and to several disordered $$\mathrm{H_{2}O}$$ molecules (Figure \ref{figure:8MR_6MR}).

The Y cation preferentially exchanges into the Na2 site, and this is in close agreement to the previously reported Sr site for Sr-ETS-4 (Braunbarth 2000a). Attempts were made to model Y into the Na1 site, but least squares refinements would not converge, and so further efforts to forcefully model Y into the Na1 site were abandoned. For the penultimate crystal structure refinements, Na1 and the split Y2 site (Y2A, Y2B, Y2C) occupancies were freely refined. Attempts were made to forcefully charge balance the crystal structure by fixing site occupancies so that their total charge sum was equal to 2, but this resulted in a worse overall fit to the data with Rwp $$>$$ 20. The sum of the Y2 occupancies was 0.115(5), and is only slightly less then the expected occupancy (0.16) for full Y exchange into the Na2 site. We are aware of the indication of OH groups on the apical O7 site of the $$\mathrm{TiO_{5}}$$ polyhedra after exchange (Ti2-O7 bond lengths before- and after-exchange = 1.67Å and 1.96Å, respectively), and the bond valence sum decreased from 4.17 v.u. to 3.63 v.u. The change in bond length and bond valence sums may indicate the presence of OH at the O7 site, and may affect the final proposed chemical formula and Y2 occupancies. The increased H component, coupled with the decreased $$\mathrm{H_{2}O}$$ could be a result of hydrolysis caused by Y (Klungness 2000) in the channels during exchange. However, we do not have direct measurements of the chemical occupancy for the H sites (or pH of the solutions), and therefore have decided to omit the influence of H chemical occupancy on the Y2 or Na1 values during the structure refinements. The final crystal structure of modeled atomic positions should be independent of the modeling (or calculation) of the H component. The ultimate goal of this study was to locate the positions of the Y and Na sites, and future work will be needed to determine the occupancies and orientation of the OH and $$\mathrm{H_{2}O}$$ sites.

Given the higher charge, larger electronegativity, and smaller ionic radius of $$\mathrm{Y^{3+}}$$ (as compared to $$\mathrm{Sr^{2+}}$$ (Braunbarth 2000)), there might exist enhanced catalytic and selective gas separation capabilities (at ambient or elevated temperatures) of REE exchange titanium silicates. Further work is needed to confirm this hypothesis and the functional role of REEs in ETS-4.

Events in the Raman Spectra

To better understand the dynamics of the ion exchange process, time-resolved Raman spectroscopy was used to monitor the molecular changes in situ. Even though all possible Raman active modes for Na-ETS-4 could not be distinguished in the collected Raman spectrum due to significant peak broadening and peak overlap (Figure \ref{fig:figure_contour-plot} and \ref{fig:figure_difference-peak-evolution}) we were able to fit track the movement of major peaks corresponding to titania, silica, 3MR, and chain vibrations. Based on past molecular vibrational interpretations of Raman mode analyses (Table \ref{table:raman_modes}), and how those vibrational modes changed as Y exchanges into the crystal structure, a model of the Y exchange processes in Y-ETS-4 is developed. The following discussion describes the changes in the Raman spectra during the ion exchange process, and it is broken up into four events that roughly indicate the onset of major changes in the observed Raman spectra. Events timing and duration was determined by taking the first and second derivatives of each peak evolution curve.

Between events 0 to 1

The processes occurring between the onset of the time-resolved experiment and event 1 (at minute 4) (Figure \ref{fig:figure_contour-plot} and \ref{fig:figure_difference-peak-evolution}), are indicated by major movement of the initial v8 peak and v9 peak (see Table \ref{table:raman_modes}) to higher wavenumbers, and minor changes for the v10 peak to lower wavenumbers (see Table \ref{table:raman_modes}). These changes indicate distortions in the octahedral geometry of the $$\mathrm{TiO{{}_{6}}}$$ groups, and may be interpreted as the $$\mathrm{TiO{{}_{6}}}$$ octahedra are bending closer together as the Ti-O-Ti bending angle of the $$\mathrm{TiO{{}_{6}}}$$ chains becomes more acute, and the internal O-Ti-O bends distort as the octahedra move closer, forcing a charge repulsion between the Ti of adjacent octahedra (Figure \ref{fig:figure_difference-peak-evolution}). The square-plane of the $$\mathrm{TiO_{6}}$$ octahedra is shown to be lengthening as indicated by the decrease in the v10 peak (Figure \ref{fig:figure_difference-peak-evolution}). The movement of the v10 peak is small in comparison to other major changes occurring.

Between events 1 to 2

The processes occurring from minutes 4 to 9 (Figure \ref{fig:figure_difference-peak-evolution}), are indicated by small changes of the v10, while all other peaks showed very small changes (<1 cm$${}^{-1}$$). The shift of the v10 from 680 cm$${}^{-1}$$ to 677 cm$${}^{-1}$$ further shows an elongation of the square-plain $$\mathrm{TiO{{}_{6}}}$$ octahedral bonds, which is further emphasized in the next event. The relative paucity of this stage during ion exchange is unknown, but could have been caused by hydration changes in the structure (similar to Cs exchange into CST (Celestian 2010)). It was not possible to measure $$\mathrm{H_{2}O}$$ Raman modes with this 780nm laser.

Between events 2 and 3

The processes occurring between minutes 9 and 16 (Figure \ref{fig:figure_difference-peak-evolution}), are indicated by major changes in the v10 peak from 677 cm$${}^{-1}$$ to 665 cm$${}^{-1}$$, happening simultaneously with v13 and v14 peak increasing from 870cm$${}^{-1}$$ to 889 cm$${}^{-1}$$ and 905 cm$${}^{-1}$$ to 925 cm$${}^{-1}$$, respectively. In addition, there were also changes occurring in the v12 peak from 786 cm$${}^{-1}$$ to 790 cm$${}^{-1}$$. Minor changes in the v12 peak from 758 cm$${}^{-1}$$ to 763 cm$${}^{-1}$$. The v11 peak also showed changes, but because of the significant peak overlap with the v12 peak, the peak fitting was unstable, and we were unsuccessful in modeling continuous trends for the v11 peak. Throughout the entire experiment, the very strong v3 peak undergoes its only change during this time as it decreases from 238 cm$${}^{-1}$$ to 235 cm$${}^{-1}$$. The changes occurring between events 2 and 3 likely indicate the continued ion exchange of Y into the structure, with the possible onset of the splitting in the Y2 site as observed in the SCXRD data. The vibrational modes associated with $$\mathrm{TiO_{6}}$$ bending (v8-v14) continued to change in similar fashion as in event 0 to 1 time-frame.

Between events 3 to 4

The processes occurring from minutes 16 to 23 (Figure \ref{fig:figure_difference-peak-evolution}), are indicated by changes in the v6, v7, v8, and v9 that are related to changes in the 3MR and to the $$\mathrm{TiO_{6}}$$ internal vibrational modes (Table \ref{table:raman_modes}). These changes are dominated by internal octahedral and square-pyramidal bending and stretching (v6 - v9). The $$\mathrm{TiO_{6}}$$ chain showed little, if any, change during this time-frame. To account for these subtle changes, it is likely that Y is continuing to increase in occupancy, but maintains the same crystallographic position. If the Y site location were to change, it is expected that changes in the $$\mathrm{TiO_{6}}$$ chains would also occur, giving rise to larger peak shifts in the Raman spectrum for the v8 and v9 peaks, but these were not observed.

After event 4

After minute 23 (Figure \ref{fig:figure_difference-peak-evolution}) significant peak shift to higher wavenumbers occur for peaks v10 and v12 that are related to stretching modes of the $$\mathrm{TiO_{6}}$$ chains. The v7 peak continues to shift to lower wavenumbers. In ETS-4, the extra-framework Na2 site is closest (2.5 Å) to the bridging $$\mathrm{O^{2-}}$$ on the $$\mathrm{TiO_{6}}$$ chains and 3MR. The Na1 site is further away (3.98 Å) from the chains. From this, it is believed that the continued cation exchange of Y into the split crystallographic location nearest of the Y2 sites (Y2A, Y2B, Y2C) are likely dominating the exchange process near the end of the observed ion exchange experiment.

Conclusions

From joint Raman spectroscopy and SCXRD for the Na-ETS-4 and Y-ETS-4 structures, we were able to model the Y exchange process and the resulting molecular changes in the host framework. Y initially exchanged into the Na2 site (transparent yellow sphere in Figure \ref{figure:8MR_6MR}), and then moved and split further away from the Na2 site (shift approximately 0.5 Å) as the occupancy of Y2 increased (Figure \ref{figure:8MR_6MR}). The results from Raman spectroscopy suggest framework changes included shortening of the $$\mathrm{TiO_{6}}$$ chains (chains are parallel to [100]), followed by elongation of the $$\mathrm{TiO_{6}}$$ chains. The shortening of the chains were likely due to the initially long Y-O bond lengths at the start of ion exchange. As Y occupancy increased, the Y2-site moved further away from the framework and the Y-O bond increased to approximately 2.6 Å. A similar process of cation migration during ion exchange has been observed for Sr$${}^{2+}$$ into synthetic sitinakite (Kramer 2012). The space-group does not change during ion exchange in ETS-4, however, unit-cell parameters of the before, and after Y ion exchange, show contraction of the b- and c-axes.

Supporting Information

Supporting Information Available: Crystallographic information file for Y-ETS-4. This material is available free of charge via the Internet at http://pubs.acs.org.

Awknowledgements

This work was in part supported by the Natural History Museum of Los Angeles County Trelawney Endowment, and the American Chemical Society Petroleum Research Fund ACS PRF 50927-UNI10.

\label{fig:figure_fitted-raman-spectrum}Results of Peak fitting of ETS-4. Peaks are labeled v1 through v20, with each fitted curve under the spectrum. Dashed red line is the fit to the data (solid blue line). Fitted regions are delineated by vertical dashed black lines and are labeled A, B, and C. For the time-resolved Raman spectroscopy data, the v1, v2, v11, v18, v19, and v20 peaks were too weak to be fitted precisely for all datasets, and therefore have been removed from that analysis.

\label{fig:figure_zorite_100}The crystal structure of Y-ETS-4 as viewed down [100]. Titania ($$\mathrm{TiO_{6}}$$, $$\mathrm{TiO_{5}}$$) are light-blue polyhedra, $$\mathrm{SiO_{4}}$$are dark-blue tetrahedra, $$\mathrm{Na^{1+}}$$ are yellow, $$\mathrm{Y^{3+}}$$ are green, and unbound $$\mathrm{O^{2-}}$$ (red) are modeled as $$\mathrm{H_{2}O}$$. Si-O 8MR are seen in the plan of the figure, as well as 6MR occupied by Na1. H atoms sites not determined and are not shown.

\label{figure:ti-nano-wires}The crystal structure of Y-ETS-4 as viewed down [001]. See Figure \ref{fig:figure_zorite_100} for color scheme. The 7MR are occupied by $$\mathrm{Y^{3+}}$$, and the titania chains are seen in the plane of the figure oriented parallel to [100]. The disordered $$\mathrm{SiO_{4}}$$ (around the Si2 site) are also shown bound to the disordered $$\mathrm{TiO_{5}}$$. H atom sites not determined and are not shown.

\label{figure:8MR_6MR}The silica 8MR and mixed polyhedra 6MR. The $$\mathrm{H_{2}O}$$ sites in Na-ETS-4 (labeled as Zow, transparent red circles, in the 8MR of this viewing direction) were searched for after Y exchange, but not found in the Y-ETS-4 structure. Arrow next to Zow indicate the possible direction of $$\mathrm{H_{2}O}$$ migration to hydrate the Y2 site. Transparent yellow circles indicate the position of the Na2 site in Na-ETS-4 prior to ion exchange. Arrows at the Na2/Y2 site illustrate the proposed direction of Y2 splitting and movement direction as indicated by Raman spectroscopy. H atoms sites not determined and are not shown.

\label{figure:local_Y2_Na1}Local bonding geometry of the Y2 (only Y2C shown for clarity) and Na1 sites, and the structure of the 3MR (Ti1-O3-Si1-O3-Ti1). H atoms sites not determined and are not shown.

\label{fig:figure_contour-plot}A contour plot for the time-resolved data of Y exchange into Na-ETS-4. The spectrum at time 0 mins. at the bottom of plot is for Na-ETS-4.

\label{fig:figure_difference-peak-evolution}Evolution of peak positions as a function of time during the Y exchange into Na-ETS-4. Plots are shown as difference Raman Shift relative to their original starting position (see Table \ref{table:raman_modes}). Lines are labeled with their vibrational mode label and their original starting position in cm$${}^{-1}$$. Major changes in the peak evolution paths are shown as events 0 through 4 (see text for descriptions).

Temperature/K 298 orthorhombic Cmmm 7.1628(3) 23.1657(9) 6.9267(2) 90 90 90 1149.36(7) 1 2.064 697.0 0.128 x 0.025 x 0.014 MoK$$\alpha$$ ($$\lambda$$ = 0.71073 Å) 3.516 to 61.922 -10 $$\leq$$ h $$\leq$$ 10, -33 $$\leq$$ k $$\leq$$ 33, -10 $$\leq$$ l $$\leq$$ 9 15287 1076 [Rint = 0.0413, Rsigma = 0.0170] 1076/0/87 1.188 R1 = 0.0561, wR2 = 0.1754 R1 = 0.0629, wR2 = 0.1821 0.94/-1.20
 a (Å) b (Å) c (Å) Reference Na-Zorite 7.238(4) 23.241(7) 6.955(4) (Sandomirskii 1979) Pb-Zorite 7.161(3) 23.22(1) 6.980(3) (Zubkova 2006) Sr-Zorite 7.23810(33) 23.1962(12) 6.96517(31) (Braunbarth 2000) Na-ETS-4 7.1751(11) 23.2272(4) 6.9727(6) (Nair 2001a) Sr-ETS-4 7.2259(13) 23.1900(5) 6.9699(13) (Nair 2001a) Y-ETS-4 7.1628(3) 23.1657(9) 6.9267(2) current study
\label{table:raman_modes}Summary of Raman Mode Analyses, * from this study. Peaks v16, v17 were expected based on comparative analysis of similar minerals from the WURM project, but not observed in this study.
Initial Position Label Relative Intensity Vibrational Mode Reference
(cm$${}^{-1}$$)
120 v1 vw lattice *
200 v2 vw Si-O-Ti bnd similar to ETS-10, \cite{Mihailova_1996}
238 v3 vs Si-O-Ti bnd$${{}_{oct}}$$, out-of-plane rock of Ti1-O4-Ti1 chains \cite{Mihailova_1996, Nair_2001}
268 v4 s Ti-O-Ti bnd$${{}_{pyr}}$$, Na translations *, similar to lorenzenite \cite{Caracas_2011}
330 v5 m Ti-O-Ti bnd$${{}_{oct}}$$ \cite{Armaroli_2000, Mihailova_1996}
440 v6 m square plane O-Ti-O bnd$${{}_{oct}}$$, Si1-O3-Ti1 D3MR bnd \cite{Mihailova_1996, Nair_2001}
510 v7 s Ti1-O3-Si1-O3-Ti1 D3MR sym str \cite{Nair_2001}
520 v8 s Ti1-O3-Si1-O3-Ti1 D3MR sym str, square plane Ti-O-Ti & O-Ti-O bnd$${{}_{oct}}$$ \cite{Nair_2001, Mihailova_1996}
545 v9 w apical O-Ti-O bnd$${{}_{oct}}$$ *
686 v10 w square plane Ti-O sym str \cite{Mihailova_1996}
710 v11 vw square plane $$\mathrm{O^{2-}}$$ asm str ∥ $$\mathrm{TiO_{6}}$$ chains *, similar to lorenzenite \cite{Caracas_2011}
758 v12 s Ti1-O4-Ti1-O4 str$${{}_{oct}}$$ chains \cite{Southon:2002ok, Nair_2001, Ferdov:2008ft, Mihailova_1996}
870 v13 w Ti-O-Ti str, apical Ti-O str$${{}_{pyr}}$$ \cite{Nair_2001, Ferdov:2008ft}, similar to fresnoite \cite{Caracas_2011}
905 v14 w Si-O str similar to: \cite{Mihailova_1996}
940 v15 vw Si-O str similar to: \cite{Mihailova_1996}
$$\approx$$ 960 v16 expected, but not observed isolated Si-O-Ti asm str lorenzenite, lintisite, \cite{Caracas_2011}
$$\approx$$ 985 v17 expected, but not observed isolated Si-O; Si-O-Ti lorenzenite, lintisite, \cite{Caracas_2011} \cite{Su:2000lz}; \cite{Ricchiardi2001}
1050 v18 vw Si-O asym str \cite{Mihailova_1996, Nair_2001}
1114 v19 vw Si-O-Si asm str \cite{Nair_2001}
1327 v20 vw unknown *

References

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